The traditional reliability of telecommunication systems that users have come to expect and rely upon is based in part on the systems' operation with redundant equipment and power supplies. Telecommunication switching systems, for example, route tens of thousands of calls per second. The failure of such systems, due to either equipment breakdown or loss of power, is unacceptable since it would result in a loss of millions of telephone calls and a corresponding loss of revenue.
Power plants, such as battery plants, address the power loss problem by providing the system with an energy reserve, i.e., a backup battery, in the event of the loss of primary power to the system. A battery plant generally includes a number of battery cabinets with backup batteries, rectifiers and other power distribution equipment. The primary DC power is produced by the rectifiers, which typically convert AC commercial voltage into a DC voltage to power the load equipment as well as to charge the backup batteries. The primary power may, however, become unavailable due to an AC power outage or the failure of one or more of the rectifiers. In either case, the backup batteries then provide power to the load.
A battery plant that powers telecommunications systems such as transmission and switching systems in wireless base stations commonly employs valve-regulated lead-acid (VRLA) batteries as the energy reserve. The backup batteries are typically coupled directly to the output of the rectifiers and may provide power to the load in the event an AC power outage occurs. During normal operation, the backup batteries are usually maintained in a substantially fully-charged state to extend a duration for which the backup batteries can provide energy to the load equipment.
With the increasing trend toward distributed power systems, the battery reserve systems are often located in outdoor uncontrolled environments. Over a decade of experience in using VRLA batteries in outdoor environments has clearly shown that high temperatures drastically reduce the life of the batteries. The lifetime of a typical VRLA battery with a rated life of ten years at a constant operating ambient temperature of 25° C. will be reduced by a factor of two for approximately every 7° C. to 10° C. rise in average operating temperature. When deployed in outdoor environments, the batteries are generally placed in closed cabinets with poor heat-exchange characteristics. The batteries are, therefore, often exposed to high temperatures with poor ventilation. As a result, a ten-year rated battery may have its lifetime reduced to a quarter or to a third of its rated value, especially in warmer climates such as Dallas, Tex.
While reducing the temperature of the operating environment of the battery is an important factor in sustaining the life of the battery, there are other ancillary considerations as well. The system employed to maintain the battery in a state of readiness (i.e., fully-charged) is another important consideration in battery reserve systems. A known technique to improve the life of a battery is to employ an intermittent charging system. An intermittent charging system is disclosed in A New Concept: Intermittent Charging of Lead Acid Batteries in Telecommunication Systems, by D. P. Reid, et al. (Reid), Proceedings of INTELEC 1984, pp. 67–71, which is incorporated herein by reference.
Since the commercial AC power source is typically available about 99.9% of the time, the battery is conventionally maintained in a float mode wherein the battery is fully charged and is essentially being topped-off continuously. With an intermittent charging system, the battery is only charged a fraction of the time and, otherwise, the battery is disconnected from the charging circuit. Such a system is very sensible with VRLA batteries especially in view of the fact that VRLA batteries suffer from relatively low self-discharge rates (e.g., less than 10% over a 180 day period at about 25° C.).
Analogous to the loss of battery capacity at higher temperatures, it is estimated that the self-discharge rate approximately doubles for every 10° C. rise in temperature. Even with the increase in self-discharge rates associated with higher operating temperatures, a relatively low duty cycle (i.e., ratio of the charging time to total time) is sufficient to maintain the battery in a state of readiness should the commercial power source be interrupted. The reactions that diminish battery life during float charging are accelerated at higher temperatures thereby further contributing to the degradation of the life of the battery.
U.S. Pat. No. 6,123,266 to Bainbridge, et al. (Bainbridge) describes a “Cooling System for Stand-Alone Battery Cabinets,” which is incorporated herein by reference. While providing a simple fan and fan control unit that moves cooling air through the battery cabinet, Bainbridge has many shortcomings. The fan control unit of Bainbridge is designed solely to turn the fan ON whenever the outside air temperature (synonymously referred to as “OAT”) is cooler than the inside cabinet temperature (synonymously referred to as “ICT”), and to turn the fan OFF whenever the inside cabinet temperature is less than the outside air temperature. No provision is made for a temperature lag, i.e., a ΔT (ICT−OAT); thus, the fan control unit of Bainbridge will cycle ON once the outside air temperature is less than the inside cabinet temperature and, as soon as the outside air temperature exceeds the inside cabinet temperature, the fan control unit will turn the fan OFF. This cycle will repeat continuously as the inside cabinet temperature varies, resulting in potentially rapid ON/OFF/ON cycling of the fan. Of course, this repeated cycling is detrimental to the operation of the fan.
While commenting that “batteries have a large thermal mass and relatively long thermal time constant,” Bainbridge makes no provision for considering the conditions wherein the inside cabinet temperature is 25° C. and the outside air temperature is less than 25° C., i.e., a period when it is probably unwise to run the fan. Running the fan under these conditions will only cool the battery further, and reduce its electrical current capacity (ampacity).
Additionally, Bainbridge makes the observation that “batteries are known to generate hydrogen as they are used.” In actuality, the majority of hydrogen is produced when the batteries are charged, not when they are being discharged. That is, the electrolytic charging process drives off hydrogen from H+ ions in the water medium of the acid electrolyte. Bainbridge, by describing outside cooling air inlet louvers near the top of the cabinet and air exhaust louvers near the bottom of the cabinet, also fails to recognize that hydrogen is lighter than air, and will therefore rise to the top of the cabinet, relying on suction from the fan at the bottom of the cabinet to draw the hydrogen from the top of the cabinet out through the air exhaust louvers. In fact, the preferred embodiment of FIG. 1 of Bainbridge shows several locations within the cabinet under the top and below each shelf that can become dead air spaces trapping the hydrogen with potentially explosive results.
Furthermore, Bainbridge makes no provision for those conditions when outside air temperature is greater than the inside cabinet temperature and the battery temperature exceeds 25° C. Bainbridge ignores the fact that a constant trickle charge is generally applied to the batteries at all times, with the voltage applied varying depending upon either outside air temperature or the inside cabinet temperature. This can result in charging of the batteries when the outside air temperature is much greater than 25° C. and will therefore produce more hydrogen. Thus, if the outside air temperature is greater than the inside cabinet temperature, Bainbridge will turn the fan OFF, and the hydrogen will accumulate in the battery cabinet, creating a potentially dangerous condition.
In addition, the charging voltage also affects the hydrogen generation rate. The relationship is: hydrogen generation is directly proportional to a higher charging voltage thereby resulting in increased hydrogen generation.
Accordingly, what is needed in the art is a recognition that maintaining a battery cabinet inside cabinet temperature is not the only important factor, and the charging voltage of a battery at temperatures well above, or well below, 25° C. can result in creation of more hydrogen gas. More particularly, what is needed is a battery cabinet environmental control system that overcomes the above-stated deficiencies in the prior art relating to both battery longevity and environmental safety.